1. Field of the Invention
The present invention relates to a display device and, more particularly, to a display device having a memory function, e.g., a display device using a ferroelectric liquid crystal element.
2. Related Background Art
A known liquid crystal element using a liquid crystal compound comprises scanning and signal electrodes arranged in a matrix form, and a liquid crystal compound filled between the electrodes to constitute a large number of pixels, thereby displaying image information.
According to a conventional time-divisional method of driving such a display element, voltage signals are sequentially and periodically applied to the scanning electrodes, and predetermined information signals are parallely applied to the signal electrodes in synchronism with the scanning electrode operations. According to the above-mentioned display element and its driving method, it is difficult to increase both the pixel density and the screen size.
The most popular liquid crystal element is a TN (twisted nematic) element since it has a relatively short response time among the liquid crystal materials and low power consumption. In a state when no electric field is applied, twisted nematic liquid crystal molecules having positive dielectric anisotropy have a twisted structure (helical structure) in a direction of thickness of a liquid crystal layer, as shown in FIG. 41A. The liquid crystal molecules of the respective molecular layers are twisted and parallel to each electrode surface between the upper and lower electrodes. However, as shown in FIG. 41B, in an electric field, the nematic liquid crystal molecules having positive dielectric anisotropy are oriented in the direction of the electric field, thereby causing optical modulation. When a display element is arranged in a matrix electrode structure by using such a liquid crystal material, a signal voltage higher than a threshold value required for orienting the liquid crystal molecules in a direction perpendicular to each electrode surface is applied to a selected area (i.e., a selected point) as an intersection between the corresponding scanning and signal electrodes. The signal voltage is not applied to non-selected intersections (non-selected points) between the non-selected scanning and signal electrodes. Therefore, in these points, the liquid crystal molecules are twisted and parallel to each electrode surface. When linear polarizers in a relationship of crossed nicols are arranged on the upper and lower surface of this liquid crystal cell, light is not transmitted at the selected point(s), but light is transmitted at the non-selected point(s) due to the twist structure of the liquid crystal and an optical rotary power, thereby providing an imaging element.
With a matrix electrode structure, a limited electric field is applied to an area (so-called "semi-selected point") where the scanning electrode is selected and the signal electrode crossing this scanning electrode is not selected, and vice versa. If a difference between the voltage applied to the selected point and the voltage applied to the semi-selected point is sufficiently large, and a voltage threshold required for vertically aligning the liquid crystal molecules with respect to the electrode surface can be set to an intermediate value between the above voltages, the display element can be normally operated.
When the number (N) of scanning lines is increased in the above system, a duration (i.e., a duty ratio) for which an effective electric field is applied to one selected point during scanning of one frame is decreased at a rate of 1l/N. For this reason, a difference between voltages, i.e., effective values, applied to the selected and non-selected points upon repetition of the scanning cycle is decreased when the number of scanning lines is increased. As a result, a decrease in image contrast and a crosstalk phenomenon cannot be avoided.
The above phenomena inevitably occurs when a liquid crystal without a bistable state (i.e., liquid crystal molecules are stably oriented in a direction parallel to the electrode surface and their orientation is changed in a direction perpendicular to the electrode surface during an effective application of the electric field) is driven by utilizing an accumulation effect as a function of time (i.e., scanning is repeated). In order to solve this problem, various driving schemes such as a voltage averaging scheme, a 2-frequency driving scheme, and a multiple matrix scheme are proposed. However, none of these conventional schemes are satisfactory. Therefore, a large screen and a high packing density of a display element cannot be obtained since the number of scanning lines cannot be sufficiently increased.
In order to solve the above problem, the present applicant filed a U.S. Ser. No. 598,800 (Apr. 10, 1984) entitled as a "Method of Driving Optical Modulation Device". In this prior art, the present applicant proposed a method of driving a liquid crystal having a bistable state with respect to an electric field. An example of the liquid crystal which can be used in the above driving method is preferably a chiral smectic liquid crystal, and more preferably a chiral smectic C-phase (SmC*) or H-phase (SmH*).
The SmC* has a structure in which liquid crystal molecular layers are parallel to each other, as shown in FIG. 42. A direction of a major axis of each molecule is inclined with respect to the layer. These liquid crystal molecule layers have different inclination directions and therefore constitute a helical structure.
The SmH* has a structure in which the molecular layers are parallel to each other, as shown in FIG. 43. A direction of a major axis of each molecule is inclined with respect to the layer, and the molecules constitute a six-direction filled structure on a plane perpendicular to the major axis of the molecule.
The SmC* and SmH* have helical structures produced by the liquid crystal molecules, as illustrated in FIG. 44.
Referring to FIG. 44, each liquid crystal molecule e3 has electrical bipolar moments e4 in a direction perpendicular to the direction of the major axis of the molecule e3. The molecules e3 move while maintaining a predetermined angle .theta. with respect to the Z-axis perpendicular to a layer boundary surface e5, thereby constituting a helical structure. FIG. 44 shows a state when a voltage is not applied to the liquid crystal molecules. If a voltage exceeding a predetermined threshold voltage is applied to the X direction, the liquid crystal molecules e3 are orientated such that the electrical bipolar moments e4 are parallel to the X-axis.
The SmC* or SmH* phase is realized as one of the phase transition cycles caused by changes in temperatures. When these liquid crystal compounds are used, a proper element must be selected in accordance with the operating temperature range of the display device.
FIG. 45 shows a cell when a ferroelectric liquid crystal (to be referred to as an FLC hereinafter) is used. Substrates (glass plates) el and el' are coated with transpatent electrodes comprising In.sub.2 O.sub.2, SnO.sub.2 or ITO (indium-tin oxide). An SmC*-phase liquid crystal is sealed between the substrates el and el' such that liquid crystal molecular layers e2 are oriented in a direction perpendicular to the substrates el and el'. The liquid crystal molecules e3 represented by thick lines have bipolar moments e4 in directions perpendicular to the corresponding molecules e4. When a voltage exceeding a predetermined threshold is applied between the substrates el and el', the helical structure of the liquid crystal molecules e3 is changed such that the directions of orientation of the liquid crystal molecules e3 are aligned with the direction of the electric field. Each liquid crystal molecule e3 has an elongated shape and exhibits refractive anisotropy in the major and minor axes. For example, when polarizers having a positional relationship of crossed nicols with the orientation direction are arranged on the upper and lower surfaces of the upper and lower glass plates, it is readily understood that there is provided a liquid crystal optical modulation device having optical characteristics which change in accordance with the polarities of the applied voltage.
When the thickness of the liquid crystal cell is sufficiently small (e.g., 1 .mu.m), the helical structure of liquid crystal molecules cannot be established even if an electric field is not applied thereto, and the bipolar moment P or P' is directed upward or downward, as shown in FIG. 46. When an electric field E or E' (the fields E and E' having different polarities) exceeding the predetermined threshold value is applied to this cell for a predetermined period of time, the bipolar moment is directed upward or downward so as to correspond to the electric field vector of the electric field E or E'. Therefore, the liquid crystal molecule is oriented in a first stable state f3 or a second stable state f3'.
Use of such an FLC in an optical modulation element has the following two advantages. First, the resultant optical modulation element has a very short response time (1 usec to 100 .mu.sec), and second, the liquid crystal molecule orientation has a bistable state.
The second point will be described with reference to FIG. 46. When the electric field E is applied to the liquid crystal molecules e3, the liquid crystal molecules e3 are oriented in the first stable state f3. This state is kept stable even if the electric field is withdrawn. When the electric field E' having a polarity opposite to that of the electric field E is applied, the liquid crystal molecules e3 are orientated in the second stable state f3'. This state is kept unchanged even if the electric field E' is withdrawn. Therefore, the liquid crystal molecules e3 have a memory function. If the level of the electric field E does not exceed the predetermined threshold value, the orientation state of the molecule is maintained.
In order to obtain a short response time and an effective memory function, the thickness of the cell is preferably minimized, generally, to 0.5 .mu.m to 20 .mu.m and, more preferably, to 1 .mu.m to 5 .mu.m.
A method of driving the FLC will be described with reference to FIGS. 47 to 49D.
FIG. 47 is a cell arrangement having a matrix electrode structure containing an FLC compound (not shown) therein. The cell arrangement includes scanning electrodes com and signal electrodes seg. An operation when the scanning electrode coml is selected will be described.
FIGS. 48A and 48B show scanning signals, in which FIG. 48A shows an electrical signal applied to the scanning electrode com1 and FIG. 48B shows an electrical signal applied to other scanning signals (i.e., the non-selected scanning electrodes) com2, com3, com4, . . . . FIGS. 48C and 48D show information signals, in which FIG. 48C shows an electrical signal applied to the selected signal electrodes seg1, seg3, and seg5, and FIG. 48D shows an electrical signal applied to the non-selected signal electrodes seg2 and seg4.
Time is plotted along the abscissa in each chart of FIGS. 48A to 48D and FIGS. 49A to 49D and voltage values are plotted along the ordinate in each chart of FIGS. 48A to 49D. For example, when a motion image is to be displayed, the scanning electrodes com are sequentially and cyclically selected. If a threshold voltage for giving the first stable state in a liquid crystal cell having bistable characteristics with respect to a predetermined applied voltage time .DELTA.t1 or .DELTA.t2 is given as -Vth1, and a threshold current voltage for giving the second stable state therein is given as +Vth2, the electrode signal applied to the selected scanning electrode com (coml) is an alternating voltage which is set at 2 V in a phase (time) .DELTA.t1 and -2 V in a phase (time) .DELTA.t2 as shown in FIG. 48A. When electrical signals having a plurality of phase intervals and different voltage levels are applied to the selected scanning electrode, an immediate change occurs between the first stable stage corresponding to the optically "dark" (black) state and the second stable state corresponding to the optically "bright" (white) state.
As shown in FIG. 48B, the scanning electrodes com2 to com5, . . . are set at an intermediate potential of the cell applied voltage, i.e., a reference potential (e.g., a ground state). The electrical signal applied to the selected signal electrodes seg1, seg3, and seg5 is given as V, as shown in FIG. 48C. The electrical signal applied to the non-selected signal electrodes seg2 and seg4 is given as -V, as shown in FIG. 48D. Therefore, the above voltage values are set to be desired values satisfying the following conditions: EQU V&lt;Vth2&lt;3V EQU -3V&lt;-Vth1&lt;-V
Waveforms of voltages applied to pixels A and B (FIG. 47) of the pixels applied with the above electrical signals are shown in FIGS. 49A and 49B, respectively. As is apparent from FIGS. 49A and 49B, a voltage 3V exceeding the threshold value Vth2 is applied in the phase .DELTA.t2 to the pixel A located on the selected scanning line. A voltage -3V exceeding the threshold value -Vth1 is applied in the phase -t1 to the pixel B on the same selected scanning line. Therefore, when the signal electrode on the selected scanning line is selected, the liquid crystal molecules are oriented in the first stable state. However, when the signal electrode on the selected scanning line is not selected, the liquid crystal molecules are oriented in the second stable state.
As shown in FIGS. 49C and 49D, the voltage applied to all pixels on the non-selected scanning line is V or -V. In either case, the voltage does not exceed the corresponding threshold voltage. The liquid crystal molecules in each pixel excluding the ones on the selected scanning line do not change their orientation state and are kept in the state established by the previous scanning cycle. In other words, when the scanning line is selected, one-line signal write is performed. The signal state is kept unchanged until the next selection is started upon completion of one frame. Therefore, even if the number of scanning electrodes is increased, the selection time/line is not almost changed, and a decrease in contrast does not occur.
As has been described above, in order to solve the problems posed by the conventional display elements using a TN liquid crystal, an FLC which has a bistable effect with respect to an electric field and allows an arrangement of a display element for maintaining the stable state is proposed. Regarding drive control of a display element using an FLC, some problems on characteristics still remain unsolved.